Amplifier



Ot. 1o, 1939.

c. w. HANSELL AMPLIFIER Filed May 27, 1931 3 Sheets-Sheet l IE/ICY VUAGE wwmwmwwwml Oct. 10, `19397. c.-w. HANSELL AMPLIFIER 3 Sheets-Sheet 2 Filed May 27, 1931 INVENTOR .CLARENCE W. HANSELL BY n@ ATTORNEY Oct. 10, 1939. c. w. HANSELL 2,175,838

AMPLIFIER n I Filed May 27, 1931 3 Sheets-Sheet 5 ...Zz-'gf' 5 14 lh l Ik r -P z' 1-L:

INVENTOR CLARENCE W. HNSELL ATTORNEY Patented Oct. 10, 19.39

UNITED STATES AMPLIFIER 'Clarence W. Hansell, Port Jetrerson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 27, 1931, Serial No. 540,311

3y (Cl. 179-1715) This invention has as its principal/object, the provision of methods and means for the elimina- -tion of distortion in electron discharge device ampliers and especially the distortion due to 5 the drooping of a characteristic curve of the amplier caused by grid rectiication.

A linear characteristic for the output of an electron discharge device ampliiier relative to its input may readily be obtained when the output of l the ampliiier is kept within approximately haii of its commercial continuous wave rating. However, beyond that point, the ratio of output to input for the ampliiier is no longer a constant due to the characteristic curve of the amplier 13 which droops beyond the half rating valve.V In

the case of telephonie communication or multiplex communication, this distortion limitsI a transmitter output to approximately half of its rated output, making a great reduction in power 20 and transmitter emciency as well as increasing cost of construction and operating expenses for transmission of signals of desired power.

The drooping characteristic of an amplifier may be attributed to several causes, the most important of which and by far the most influential factor in causing the droop is that of grid rectiflcation.

Relative to other causes of distortion, such as saturation of vacuum tubes used for an amplier, grid rectiilcation pments a diflicult problem inasmuch as it causes an abrupt break in the characteristic amplier curveof the ampliiier.

That is, in the case of saturation, the saturation characteristic of tubes is fairly continuous and i can be balanced out to a great extent and practically suiiiciently, by adjustingearlier ampliiier stages so that they produce a curvature opposite to the saturation curvature to becorrected.A

Heretofore, in order to reduce the eil'ect of l grid rectiiicatlon, the input circuit or the ampliiler has been artiiicially loaded with resistances' so that the increased losses produced by the -grid rectification are small in comparison with the losses throughout the loaded input circuit. Conlequently, the change in eilective grid resistance, due to rectification, vhas a relatively small effect upon the voltage regulation of the input circuit and, therefore, upon the curvature of the ampli'- iler characteristic.

But such a loading method, in order to'be practically eective, necessitates a large ratio between radio frequency .losses in the input or grid excitation circuit relative to the grid'losses, entailing a marked reduction in power, emciency, and ampliiication in the stage or the amplifier.

As a result of this, for a given output, an increased number of stages are needed having concomitant thereto, disadvantages of increased construction and operating costs to say nothing of increased unreliability of an installation.

To' overcome the foregoing disadvantages is a further object of the present-invention and in the accomplishment thereof, according to the present invention, the input or grid circuit of an amplier is uniformly loaded up to a point where grid rectification occurs with its consequent increase in loading, and beyond that point this loading is rapidly decreased whereby the total load on the input circuit remains constant, or

may even decrease so that the inputl voltages 15.

are built up more rapidly on the grids or control electrodes of an amplifier or ampliiiers, after grid rectication occurs than was the case before my invention was applied. Y Y To accomplish such avariation in threaded@ loading, it is a further object of the present invention to provide ad electron discharge device load for the grid circuit of an ampliiier, the electron discharge device or devices forming part of the load being adjusted'so that cut oi occurs be- 25 yond the point where grid rectification occurs in the amplier. In other words, according t`o the present invention, the amplifier input circuit is loaded withl vacuum tubes instead' 'of xed resistances, the tubes being so adjusted that their 30.

quencyvoltage in such a manner as to cause a correction of the non-linear `amplitude characteristic of the electron discharge device amplilier. i

Although it-has been attempted to deiine the present invention in the appended claims, it, of course, may best be understood by referring to the accompanying drawings, given solely by way 40 of illustration, wherein;

Figure lJillustrates a characteristic curve for a kw. ampliiier, Y f

Figure 2 illustrates a transmitter utilizing an amp`1liier having a characteristic'such as shown 45 in Figure 1 and having means for correcting curvature of the ampliiier characteristic, Figures 3 and 4 are curves explanatory of the curve correction method and means of the present invention, and, Figure 5 illustrates a modification of the invention shown in Figure 2. Y v

Fig. 6 illustrates a further modiiication of the `invention shown in Fig. 2.

Tmning to Figure 1, there is graphically illusf .trated by the solid line ing tubes 20,

2, the relationship between the voltage or current output o a 50 kw. commercial amplifier and its input or grid voltage. It will be noticed that at point d, or at approximately half of. the maximum output power of which the amplifier is capable, the curve begins to droop and is no longer a straight line such as it is between point 4 and the zero point of the curve. For distortionless full rating output it is desired that the characteristic curve continue in a straight line as indicated by the dotted portion 6. To give the amplifier A of Figure 2, which normally has the drooping characteristic shown 1n Figure l, the desired linear characteristic beyond.

the point where grid rectification occurs, the amplier A, comprising pushpull connected electron discharge devices 8, IU, having a tunable input circuit I2 and a tunable output circuit I6 together with means comprising neutralizing condensers I6, I8 for preventing the eiects of interelectrode feed back, has coupled to its input circuit, as loading therefor, electron discharge devices 20, 22.

The latter have their anodes 23 coupled to the input circuit I2 of amplier A through current limiting resistances 24 and blocking condensers 26; If desired the resistances 24 can be omitted. The anodes 23 of the loading vacuum tubes 20, 22 are paralleled for unidirectional potentials supplied to lead 3B from a source of potential (not shown), by high frequency inductance coil 32 which, in addition, serially connects the anodes for high frequency currents. The grids of the loading tubes 20, 22 are polarized from a suitable source of potential (not shown) through lead 3| and they are grounded for radio frequency potentials by means of by-passing condensers 3G in series and parallel with which may be resistances (not shown) for suppressing parasitic oscillations in the loading tubes.

The anodes and grids ofthe loading tubes 20, 22 are undidirectionally polarized so that on negative half cycles of radio frequency potentials, derived from the input circuit I2 of amplifier A and applied to the anodes of the loading tubes, exceeding the peak potential before which grid rectiiication occurs, the tubes 20, 22 cut off. That is, there is cessation of current flow in the load- 22 during a negative 'portion of each cycle after a certain value of radio frequency potential is exceeded which results in an increase uniformly or in impedance of the loading tubes as the input to amplier A goes beyond the point where grid rectification' occurs and the effective grid resistance decreases.

That is, referring to Figure 3, which indicates graphically the relation between the anode current now in a particular type of loading tubes and the applied instantaneous anode, radio frequency voltages between zero and 900 volts, the anode current through the loading tubes relative to the applied radio frequency potential is substantially a 'constantor in other words, the characteristic of the loading tubes up to 900 volts corresponding to point l of Figure 1 is almost a straight line, thereby loading the input circuit I2 with an equivalent resistance of a constant value. However, beyond 900 volts, which, as already indicated, corresponds to pbint l of Figure 1 where grid rectiilcation occurs in the amplifier A, cut on occurs in the loading tubes 2l, 22 during a portion of the cycle of alternating current anode voltage, as a result of which their effective alternating current impedance or resistance increases thereby causing a discontinuous decrease in the loading o the input circuit I2 of amplifier A of the same character as the increase in loading by starting of grid rectification in the ampliiier.

This decrease in loading of the input circuit of amplier A causes voltages to be built up in the input circuit at a much faster rate after grid rectiiication occurs than would be the case if loading by xed resistances were used` Consequently, by virtue of this increased speed of building up of potentials on the input electrodes of amplifier A, the characteristic curve of amplier A will no longer droop but will continue on the straight dotted line 6 of Figure 1 thereby prolonging desired linear characteristic up to a point much nearer the maximum possible power output from the ampliiier.

Customarily, the voltage applied to the amplier grids of amplier A is a modulated voltage, derived from constant frequency carrier energy from source I combined with amplied modulating or audio potentials from source 3 in a modulator and radio frequency ampliiier 5. The current which will iiow through the anode circuits of the correcting tubes 2U, 22 is graphically illustrated in Figure 4, the shaded areas representing the current cut off by the correcting vacuum tubes. This decrease in current corresponds to the power made available for correcting the characteristic of ampliiier A.

A mod iiication of the present invention is illustrated in Figure wherein the pushpull amplifier A is of the screen grid type, the screen grids Vbeing biased through a suitable resistance 40 connected to the vplate potential lead. The screen grids, of course, prevent the eiiects of interelectrode feed back.

To increase the cut oiI effect of anode potentials unloading tubes 20, 22, a rectifier 3| is coupled to the input circuit I2 here shown inductively coupled thereto. The rectified current is applied to a resistor 33 in the grid polarizing lead 34 sov that the added potential engendered in resistance 33 is sufficient, at the time grid rectiiication occurs in amplifier A to cause the beginning of a decrease in the proportionality of the radio frequency current absorbed by the tubes 2|! and 22. This proportionality continues to decrease, as the input to the ampliner increases, in a manner to correct the objectionable ,drooping amplitude characteristic of the ampliiler.

' Of` course, if desired the rectification may be produced in the grids of tubes 20, 22 without using the separate rectifierV 3|, by feeding radio frequency energy to the grids of tubes 2U, 22 of the same polarity as their anode voltages and at the same time using a combination of iixed bias from a negative source and grid leak bias obtained by passing rectled grid current through a resistance. Figure 6 illustrates one method for doing this wherein the grids of tubes 20, 22 are connected together by a grid leak resistance 2lb, and supplied, in addition to the supply through the interelectrode capacities of the tubes, with high frequency currents through condensers 35a. Condensers 36 act as by passing condensers for the alternating component of the rectified current. Having thus described my invention, what I claim is:

1. In a system for the amplification of undulatory electrical currents, an ampliiier comprising Va. pair of pushpull connected electron discharge way as to correct the characteristic of the pushdevices having an input circuit and an output circuit, electron discharge devices coupled to the input circuit of said amplier acting as a load therefor, and means for polarizing electrodes of the devices forming the load whereby when said f amplier characteristic begins to droop, the loading electron discharge devices are cut off thereby allowing input potentials to vincrease at a faster rate on the input electrodes of'said electron discharge device amplifier.

2. In a system for the amplication of undulatory electrical currents such that the ratio of the amplied currents to the currents' to be ampliiied remains constant over a wide range of values for input currents, the combination of a pair of electron discharge devices connected in pushpull fashion for amplifying the undulatory currents,

an electron discharge device load coupled to the input circuit of the pushpull connected electron discharge device amplier, and means for polarizing electrodes of said loadelectron discharge devices so as to alter their conductivity in such a pull connected electron discharge device amplier so that the ratio of its output to its input, for high values of input, remains constant over a desired predetermined range of values for in put currents supplied to said pushpull connected electron discharge device amplifier.

3. In combination, an amplifying system comprising a pair of pushpull electron discharge devices each having an anode a cathode and a control ele'ctrode,.means for preventing oscillation generation in said devices, an input circuit connected across the control electrodes of sai'. de'- vices, a pair of load electron discharge devices coupled to said input circuit, means for simultaneouslykenergizingV said input circuit and said load devices from a source of energy to be am- 'CLARENCE W. HANSELL. 

